A key intermediate in the production of nylon is adiponitrile (ADN). ADN is commercially produced via hydrocyanation of 1,3-butadiene and 3-pentenenitrile (3PN) in the presence of a catalyst comprising nickel(0) and phosphite ligand. The phosphite ligand used commercially is a monodentate phosphite, such as a triarylphosphite, that forms nickel-ligand complexes serving as catalyst precursors for the reactions. Although useful, monodentate phosphites can result in relatively low catalyst activity and relatively high nickel consumption.
Recently, significant improvements in nickel catalyst activity and yield to 3PN and ADN have been realized by using catalysts comprising nickel(0) and bidentate phosphites as ligands. Bidentate phosphite ligands, in general, contain two phosphorus donor atoms that may form cyclic chelate structures with a single transition metal.
Bidentate phosphites, also referred to as diphosphites in this specification, of the general structure (RO)2P(OZO)P(OR)2 are of special interest. Traditionally, such diphosphites may be synthesized by a first reaction for preparing a phosphorochloridite, (RO)2PCl, from the reaction of PCl3 with ROH in the presence of a tertiary organic amine. Then in a subsequent reaction, a difunctional alcohol, such as HO—Z—OH, may react with the phosphorochloridite in the presence of additional tertiary organic amine to produce (RO)2P(OZO)P(OR)2. A function of the tertiary organic amine is to neutralize the HCl co-product in both reaction steps through the formation of a tertiary organic amine hydrogen chloride salt. The nature of ROH and HO—Z—OH as well as the conditions chosen for each reaction step may influence the selectivity to desired products, (RO)2PCl and (RO)2P(OZO)P(OR)2.
U.S. Pat. No. 5,235,113 and WO 96/22968, for example, disclose the synthesis of diphosphites. U.S. Pat. No. 5,235,113 discloses a process for the preparation of a diphosphite of structure (RO)2P(OAO)P(OR)2 where A is biphenyl and R is 3,6-di-t-butyl-2-naphthyl. WO 96/22968 discloses syntheses of multidentate phosphite compounds of the type (ArO)2P(OZO)P(OAr)2 where Ar and Z are substituted or unsubstituted aryl groups.
U.S. Pat. No. 6,069,267 provides a process for the preparation of organodiphosphites of the general formula (R1O)2P(OZO)P(OR1)2 wherein R1 and Z are different substituted or unsubstituted aryl groups
WO 2004/050588 discloses that low temperature and viscosity of the product mixture below 0° C., for example between 0° C. and −20° C., add significantly to operating cost and process complexity.
WO 2004/091780 also describes a process for preparing a crude ligand mixture comprising bidentate phosphite ligands of the structural formula (R1O)2P(OZO)P(OR1)2 by contacting a first reaction product, comprising (R1O)PCl, at a temperature between about −25° C. and about +35° C. with about one half molar equivalent of HO—Z—OH in the presence of an organic base.
Accordingly, there is a need for a simple and selective process for preparing diphosphites that overcome problems identified in these references.